1. Field of the Invention
The present invention generally relates to a touch panel and an input device therewith, and specifically relates to a touch panel that employs a resistance film, and that is provided on a screen of a personal computer, a word processor, an electronic notebook, etc. for inputting.
2. Description of the Related Art
When using a personal computer and the like equipped with a touch panel, a problem is that readability, therefore, operability is reduced by reflection of fluorescent light indoors, and the sun light outdoors. The problem has been coped with by providing a λ/4 phase-difference film and a polarizing plate on the surface of a touch panel, such that reflection of an external light is suppressed in order to enhance the readability. Here, the polarizing plate is generally structured by an extended film of dye mixed polyvinyl alcohol, which has polarization properties, and is sandwiched by films of triacetyl cellulose (TAC). The heat expansion coefficient of triacetyl cellulose (TAC) films 17b and 17c is 5.4×10−5 cm/cm/degree C., which is comparatively large. The heat expansion coefficient of the polarizing plate, due to its structure, is approximately equal to the heat expansion coefficient of the triacetyl cellulose (TAC) films.
A touch panel is exposed to the external environment, and is influenced by ambient temperature and humidity. Under a high ambient temperature, the polarizing plate provided for improvement in visibility expands greatly, causing a curvature, that is, the central part of the touch panel swells outward. If the touch panel swells, the force required for the pushing operation increases, giving a different feeling of operation and reducing operability. Therefore, resistance to ambient conditions, such as heat and humidity, is also important for a touch panel.
FIG. 1(A) shows a conventional touch panel 10 indicated by Japanese Patent 2001-34418. The touch panel 10 is structured by a touch panel main part 15 that includes a glass substrate 11, a film 12 layered on the glass substrate 11 attached by double-sided adhesive tape 13, on which a λ/4 phase-difference film 16, and a polarizing plate 17, both for suppressing reflected lights are provided, and further a polyethylene terephthalate (PET) film 18 for resisting environmental conditions is provided. On the glass substrate 11, a transparent resistance film 11a and dot-like spacers 11b are formed. Under the film 12, a transparent resistance film 12a is formed.
The polarizing plate 17 is structured by a film (polarizing film) 17a that is made by extending a film of dye mixed polyvinyl alcohol (PVA) to provide a polarization property, sandwiched by films 17b and 17c that are made from triacetyl cellulose (TAC). Thickness of the films 17b and 17c is several times the thickness of the film 17a. Thus, the heat expansion coefficient of the polarizing plate 17 becomes approximately that of the films 17b and 17c made from TAC, that is, 5.4×10−5 cm/cm/degree C.
The heat expansion coefficient of the film 18 made from PET is 1.5×10−5 cm/cm/degree C., and is about ¼ of the heat expansion coefficient of the films 17b and 17c made from TAC. The thickness of the film 18 made from PET is about the same as the thickness of the polarizing plate 17.
The film 18 made from PET pasted on the surface of the polarizing plate 17 functions such that the above-mentioned swelling phenomenon is suppressed under a severe high temperature condition.
Inventors of the present invention closely examined the touch panel 10 that contains the film 18 made from PET pasted on the surface of the polarizing plate 17, and through experiments, found the following matters relative to resistances to heat and humidity.
The touch panel was left under a high temperature and high humidity condition, then put back to the usual temperature and humidity condition. Then, the touch panel 10 was deformed into a concave shape as shown in FIG. 1(B).
If the surface of the touch panel 10 is deformed into a concave shape, there is a possibility that the film 12 will contact the glass substrate 11, causing a short circuit of the touch panel 10. If the resistance films 11a and 12a touch each other, making a short circuit, the touch panel 10 will not function properly. Therefore, concave deformation of the surface of a touch panel 10 is a more serious problem than the above-mentioned swelling phenomenon.
Further, when the surface of the touch panel 10 was deformed into a concave shape, exfoliation 19 occurred at a part of the adhesion portion of the film 12 to the glass substrate 11.
The inventors of the present invention analyzed the concave deformation and determined a cause thereof as follows.
When the polarizing plate 17 was exposed to an environment of high temperature, and then returned to a normal temperature, the polarizing plate 17 had become a little shorter than the original length.
Here, the heat contraction rate is defined as {(L0−L1)/L0}×100, where L0 is the original length, and L1 is the length after the exposure to high temperature and returning back to a normal temperature.
The heat contraction rate of the polarizing plate 17 was 0.9%, after a 30-minute exposure to a 100 degrees C. ambient environment.
Further, when the polarizing plate 17 was put in a high humidity ambient environment, and returned to the usual environment afterwards, it was noticed that the polarizing plate 17 had become a little shorter than the original length.
Then, a moisture absorption contraction rate is defined as {(L0−L2)/L0}×100, where L0 is the original length, and L2 is the length after the exposure to the humidity.
The moisture absorption contraction rate of the polarizing plate 17 was 1.5% at an exposure to 60 degrees C. and 95% RH for 100 hours. The water absorption rate of the polarizing plate 17 was 2–4.5%, when it was soaked in 23-degree C. water for 24 hours.
As above, the polarizing plate 17 expands thermally when exposed to high temperature, and shrinks to shorter than the original length when it is returned to room temperature, resulting in a permanent deformation. The polarizing plate 17 thereby becomes shorter than the film of the touch panel immediately underneath. Further, when the polarizing plate 17 is exposed to high humidity, it expands due to moisture absorption. When it is returned to room temperature, the moisture is reduced and the polarizing plate 17 contracts to shorter than the original length, resulting in a permanent deformation. The polarizing plate 17 shrinks to shorter than the film of the touch panel immediately underneath. Analysis indicates that the central part of the touch panel deforming into a concave shape is a permanent deformation, with the polarizing plate 17 shrinking to a shorter length than the film of the touch panel.
From above, it is determined that the permanent deformation of the polarizing plate caused by heat and humidity is the real problem to solve.